1. Field of Invention
The present invention relates to a method of forming an etching mask, and more particularly, to a method of forming an etching mask with a high etch selectivity of a hard mask for forming a thin film pattern to a photoresist.
2. Description of the Prior Art
Conventionally, in order to form a thin film pattern, a G-line (436 nm) resist and an I-line (365 nm) resist or a KrF (248 nm) resist are applied, and a photoresist pattern is then formed by performing a photolithography process using a mask. Thereafter, a thin film pattern is formed by performing an etching process using the photoresist pattern as an etching mask.
However, due to the reduction of the line width of a device and the limit of a photolithography process, a thin film pattern with an ultra-fine line width is currently formed using an ArF (193 nm) resist and a hard mask pattern in a line width of 80 nm or less.
FIGS. 1A to 1C are sectional views conceptually illustrating a problem of a conventional method of forming a thin film pattern using an ArF (193 nm) resist and a hard mask pattern.
Referring to FIG. 1A, a thin film 20 to be patterned is formed on a substrate 10. A hard mask film 30 and a photoresist pattern 40 are formed on the thin film 20. The hard mask film 30A is made of silicon nitride film, while the photoresist pattern 40 is made of an ArF resist. This is because the thickness of a resist is reduced due to the limit of a lithography process so that only the existing resist does not serve as a sufficient etch barrier mask. Thus, the hard mask film 30 is formed between the resist and the thin film, thus to use it as an etching mask.
Referring to FIGS. 1B and 1C, to use the hard mask film 30 as an etching mask, the hard mask film 30 is patterned by etching the hard mask film 30 using an ArF photoresist pattern 40. Thereafter, the lower thin film is patterned by performing an etching process using the patterned ArF photoresist and hard mask film 30 as an etching mask.
However, since an etch selectivity of the hard mask film 30 to the photoresist pattern 40 is low when patterning the conventional hard mask film 30 and thus a mask film is eroded, there is a problem in that the pattern of the hard mask film 30 with a desired shape is not formed.
A mixed gas in which oxygen (O2) is mixed with a fluorocarbon-based gas is used as an existing etching gas for etching the hard mask film 30. However, the etch selectivity of the silicon nitride film used as the hard mask film 30 to the ArF photoresist is in a range between 1.5:1 and 4:1 when the aforementioned mixed gas is used. Thus, there is caused a problem in that the photoresist is also removed when the hard mask film 30 is etched, or the durability of the photoresist is weakened by the mixed gas when etching the hard mask film 30 having the same thickness as the photoresist and the photoresist is collapsed.
That is, the hard mask film 30 and the thin film 20 should be patterned such that each thereof has a width identical with width A between the initial photoresist patterns 40 as shown in FIG. 1A. However, since the etch selectivity of the hard mask film 30 of the photoresist pattern 40 is low as described above, the photoresist pattern 40 is also removed when the hard mask film 30 is etched. Accordingly, a pattern of the hard mask film 30 with width B larger than the desired width A is formed as shown in FIG. 1B. Thereafter, in a case where the thin film 20 is patterned by performing an etching process using the pattern of the hard mask film 30 with the aforementioned large width as an etching mask, there is caused a problem in that a thin film pattern with a width larger than that of an initially desired shape is formed.
Further, although not shown, since the durability of the photoresist is weakened in the etching process for the pattern of the hard mask film 30, the photoresist pattern 40 is collapsed. Accordingly, there is caused a problem in that the lower hard mask film 30 is not patterned, and thus it is impossible to pattern the thin film 20.